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Solar-panel fold failure may mean extra spacewalk

By Kelly Young

This graphic shows how far the P6 solar array (two short panels in centre) was able to retract on Wednesday (Illustration: NASA)

The retraction of a six-year-old solar array on the International Space Station (ISS) did not go to plan on Wednesday. Ground controllers left it only partly folded while they consider their options for getting the device pulled back fully. The problem may mean the crew of the space shuttle Discovery has to undertake a fourth spacewalk.

“We don’t want to rush into a fix of a problem that just occurred,” says John Curry, Discovery’s lead flight director. As a result, Thursday’s spacewalk to tackle other jobs will proceed as planned, without a fix to the solar array.

Beginning at 1328 EST (1828 GMT) on Wednesday, ground teams and the astronauts inside the ISS attempted to fold one side of the 35-metre (116 feet) P6 solar array back into a box on the top of the ISS. This needed to be done so a newer P4 solar array can freely rotate to track the Sun without colliding with its neighbouring array.

After almost 6.5 hours of trying to get the array to retract and more than 40 remote-control commands sent to the array, only 13 and a half of the array’s 31 segments were safely folded in the box.

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However, this had pulled the array far enough back so the two arrays will not hit one another when P4 rotates for the first time. There are five metres of space between the two arrays at their closest points. Nonetheless, NASA wants to get the P6 array fully retracted before they move it to another part of the station on a future mission.

Pile up

There were actually two separate problems ground controllers encountered during the retraction.

First, the array behaved rather like a map that does not fold back together neatly. Some of its sections did not slide properly into the box. When this happened, they stopped the retraction, pushed the array back out a little, and then tried to retract again.

Wednesday’s showstopper was caused by one segment in particular, known as bay 17.5. It would not slide into the box, causing a pile up when subsequent segments started to roll over on top of one another. NASA tried to retract the array beyond this point four times without success.

Reconfiguration

A fourth spacewalk could add one day to the already busy mission. Shuttle and station managers are considering sending astronauts to positions at the base of P6 and somewhere down its length to help guide the array into the box.

NASA teams were meeting overnight to discuss the safety of such an operation. Astronauts Bob Curbeam and Christer Fuglesang would be the crew members asked to make the spacewalk.

The solar arrays on the P6 element of the ISS have been in space since 2000, and this was the first time they had been retracted in six years. When they were installed, they were put on “top” of the station.

The newer arrays, part of the P4 element, were installed in 2006 in their permanent position on the end of a long truss. The P6 element will eventually be moved outboard of P4 on the truss. The other side of the P6 solar array will be retracted on a shuttle mission in 2007 in advance of this move outboard, so managers are eager to learn the cause of the problem and how to prevent it in the future.

Solar flares

Despite the setback, astronauts Fuglesang and Curbeam will make their second spacewalk on Thursday, as planned. During their excursion, they will help rewire the ISS so that the station will have enough electrical power to add energy-intensive components, such as future laboratories.

Meanwhile, for the past two nights, flight surgeons have asked the crews to sleep in areas of the shuttle and station with additional shielding to protect them from increased solar activity and the possibility of extra radiation heading their way. The Sun released a large solar flare at 0240 GMT on Wednesday. Satellites were expected to show some minor problems, but the astronauts were not expected to be in any danger.

The Return of the Space Shuttle – Learn more in our continuously updated special report.